A good description of the problem which this invention addresses in the context of drilling may be found in an article by Thomas W. Beihoffer et al in the May 16, 1992 Oil & Gas Journal, page 47 et seq, entitled "Cationic Polymer Drilling Fluid Can Sometimes Replace Oil-based Mud." As stated therein, "(S)hales can become unstable when they react with water in the drilling fluid. These reactive shales contain clays that have been dehydrated over geologic time by overburden pressure. When the formation is exposed, the clays osmotically imbibe water from the drilling fluid. This leads to swelling of the shale, induced stresses, loss of mechanical strength, and shale failure." Shale crumbling into the borehole ("sloughing") can ultimately place a burden on the drill bit which makes it impossible to retrieve.
Salts such as potassium chloride have been widely used in drilling treatments to convert the formation material from the sodium form by ion exchange to, for example, the potassium form which is less vulnerable to swelling; also the use of high concentrations of such potassium salts affects the osmotic balance and tends to inhibit the flow of water away from the high potassium salt concentrations into the shale. However, it is difficult to maintain the required high concentrations of potassium salts in the drilling fluids. In addition, the physical introduction of such salts causes difficulties with the preparation of the viscosifying polymeric materials typically used for drilling. Inorganic salts can also have a harmful effect on the environment if released. While in the above cited Beihoffer et al paper, the use of cationic polymers is suggested as a supplement for the potassium salts in drilling fluids, the authors do not propose the particular polymers we use, which we have found to be especially effective because of their resistance to shear and their compatibility with anionic agents, as well as their advantageous charge density.
The reader may be interested in "The Separation of Electrolyte Effects From Rheological Effects in Studies of Inhibition of Shales with Natural Moisture Contents" by Beihoffer et al, SPE Paper 18032, which also contains a complete description of the Roll Oven Test referred to below. It is incorporated by reference.
Foaming agents commonly used in air-foam drilling generally tend to be anionic surfactants. Such foaming materials are well known and frequently are ethoxylated and sulfated, such as alcohol ether sulfates. They may be mixed with alpha olefin sulfonates, and commercially may be in solvents, including organic solvents added for freeze resistance. Polymers used in the presence of such surfactants must be compatible with them. Also, the shear forces in air drilling can be particularly high, and any polymeric additive should be able to withstand turbulent flow conditions, i.e. a Reynolds Number of up to about 500,000. Most of the contemporary technology uses acrylamide-based copolymers that shear easily when subjected to such turbulent conditions.